Recovery of h2s and nh3 from olefin cracking



United States Patent O 3,287,436 RECOVERY OF H2S AND NH3 FROM OLEFIN CRACKING Brian J. Ozero, New York, N.Y., assignor to Halcon International, Inc., a corporation of Delaware Filed Oct. 4, 1963, Ser. No. 314,047 11 Claims. (Cl. 260-680) This invention relates to processes for recovering H2S or the like catalytic material from the reaction mixture resulting from a reaction step carried out in the presence of SH radicals and an inert diluent and recycling the SH providing material together with diluent to the reaction step, and more particularly to an olefin cracking process wherein the reacted material is contacted with aqueous ammoniacal solution as a scrubbing agent to remove a mixture of H2S or the like material with ammonia Which is also an inert diluent, and recycling this mixture to the reaction step in vapor form and it relates especially to such a process for cracking six carbon atom olefins which are isoprene precursors wherein the vapor mixture to be cracked is introduced to a cracking zone at an elevated pressure, eg., of about 100 p.s.i.a. and cracked. Additionally, the sulfur content of the recycled vapor is higher than the sulfur content of the aqueous solution from which it is stripped.

The preparation of isoprene from appropriate precursor oleins in the presence of SH radicals is a commercially interesting process. This process is described in detail in British patent specification 916,133.

The olen cracking process comprises subjecting a cracking mixture comprising at least one olefin having in its molecule a carbon-to-carbon bond in a position beta to the double bond together with a source of SH radicals to elevated temperatures for periods of time suitable for cleaving the carbon-carbon bond which is in a position beta to the double bond. of said olen.

It may be carried out in any conventional manner usually employed in the art of cracking oleiins.

Generally, the conditions of cracking which .may be employed in this invention may be widely varied, depending upon the particular olefin to be cracked and the products desired. For instance, the cracking temperature may be varied from about 300 C. to about 1000 C. However, it is -usually preferred to crack olens at temperatures ranging between 500 and 800 C. and it is generally most preferred to employ temperatures ranging between 625 and 725 C.

The time that the olens are in the cracking Zone during the p-ractice of this invention may range broadly from about 0.001 to about 3 seconds. However, it is preferable that times varying from 0.05 to 1.0 second and most preferable 0.1 to 0.5 be employed. These times are referred to usually as residence times and are usually defined as the time required for one mol of incoming gas, be it pure olefin or mixtures with other oleiins or diluents, to pass through the cracking zone.

Generally, the oleins are fed to the cracking reactor either as pure olens or in mixture with other olens or in mixture with some inert diluent. It is usually desirable to employ a diluent such as steam, carbon dioxide, hydrogen or parafns such as methane, ethane, propane, butanes, penetanes, and olefins such as ethylene and protanes, pentanes, and olens such as ethylene and propylene and butene-2 and the like. These hydrocarbons do not crack at the temperatures employed to crack the olens in the practice of this invention. Of these, steam is preferred because of economy. Propane and pentane may in some cases be preferred in the place of steam.

The ratio of diluent to olefin employed in the practice of this invention may be widely varied from about 0.5/ l

ice

to about 15 1 or more mols of diluent per mol of olefin. However, if more than about a 15/1 ratio is employed the process is no longer economical. It is preferred to use a diluent to olen ratio ranging from about 2.0/1 to The olens may also be cracked without diluent.

The pressures employed in the cracking zone while cracking olens may be varied from about 10 millimeters of mercury `to 500 pounds per square inch gauge. How' ever it is preferred that the pressure range from about atmospheric to about pounds per square inch gauge with about 1 to about 2 atmospheres being most preferred.

The SH radical which is employed to increase the eiciency obtained when olefns are cracked in accordance with the practice of this invention may be provided by Various means. Hydrogen sulphide will under the conditions employed in the practice of this invention dissociate to form hydrogen and an SH radical. Low molecular weight mercaptans such as ethyl, normal propyl, isopropyl, normal butyl, isobutyl and the a-myl and hexyl mercaptans will also form an SH radical at the temperatures employed in the cracking operation. Sulphur could possible be employed. to produce SH radicals since under the conditions of the cracking there is some free hydrogen which would be available to combine with sulphur at the cracking temperature to give the SH radicals. Of these methods which may be employed normal propyl mercaptan and hydrogen sulphide are the preferred materials with which to supply SH radicals in the practice of this invention.

The SH radical employed in the practice of this invention may be used in amounts varying from about 0.5 to about 50 mol percent of SH radical based on the total mols ofthe olefin to be cracked. It has been found, however, that excellent results have been obtained by employing from about 5 to about 10 mol percent of the SH radical. It should be noted that one mol of the SH radica-l is produced for each mol of the SH radical producing compound employed.

From the commercial viewpoint it is important that sulfur-containing catalytic material be recovered and 4recycled to the reaction step. `Conventional regenerative scrubbing systems can remove H2S or the like material by stripping in a column with steam (injected directly or generated in a reboiler), recovering the H2S by condensing out the steam, and then compressing the recovered H2S for recycling. This method and equivalent systems leave much to be desired from the economic, operationaL,

and industrial waste disposal problem viewpoints. The art is confronted by the problem of providing such a recovery process which is less complicated, more efficient, and/or more economical.

The discoveries associated with the invention `and relating to solution of the above, problems, and the objects achieved inV accor-dance with the invention. as set forth.

herein include the provision of:

A process for recovering a source of SH radicals fromthe reaction product of a reaction step carried out in the presence of SH radicals as catalyst and an inert diluent and recycling the sulfur-containing material and diluent to said reacti-on step, which process comprises contacting said reaction product with aqueous ammoniacal solution whereby most of the sulfur-containing material is absorbed by said aqueous solution, separating and stripping the sulfur-rich aqueous solution with the diluent which functions las a volatile inert stripping agent to form a vapor mixture containing the source -of SH radicals toget-her with diluent and ammonia, and passing said vapor mixture to said reaction step;

Such a process wherein steam is the stripping agent and the stripping step is carried out at an elevated pressure suficiently high to give a recycled vapor mixture at a the desired inlet pressure for the elevated pressure reaction step;

Such a process wherein the reaction step is cracking an olen having in the molecule a carbon-to-carbon bond in a position beta to the double bond, the presence of from about 2 to about 10 mol percent of SH radicals;

Such a process wherein the olefin contains six carbon atoms in the molecule;

Such a process wherein the steam stripped aqueous solution is recycled as liquid for contacting the reaction product;

Such a process wherein the recycled stripper liquid is used to cool the reaction product by direct contact;

Such a process wherein the recycled stripper liquid is used to scrub SH providing material from the reaction product;

Such a process wherein the contacted reaction product is partly liquid and partly gaseous, and the latter part is partially condensed -by cooling in contact with dilute aqueous ammoniacal solution, whereby more sulfur-containing material is removed therefrom, and the resulting ammoniacal solution is included in the material subjected to the steam stripping step;

Such a process wherein the recycled vapor mixture is at about 100' p.s.i.a., the sulfur-rich Iaqueous -ammoniacal solutions are mixed together and the resulting solution contains 5 wt. percent of sulfur-containing material calculated as H2S, the vapor containing at least 2 mol percent of sulfur-containing material calculated as H2S;

Such a process wherein the content of Si-I providing material in the aqueous liquid is lower than the content required in the vapor;

Such a process wherein the concentration of SH providing material in the vapor can be easily and effectively controlled and varied by changing the iiow of aqueous solution to the stripper, rather than varying the concentration of SH in that solution;

Such a process wherein the content of SH providing material in the vapor is higher than the content thereof in the aqueous liquid;

Such a process wherein the vapor recycled contains sufcient SH to provide the concentration of SH used in the cracking step; and

Other objects which will be apparent as details or embodiment of the invention are set forth hereinafter.

The accompanying drawing is a ow diagram of a preferred system for the invention.

As shown in the drawing, the stripper 11 is fed liquid via line 13 (after heating in exchanger 16) which is contacted with high pressure steam fed via line 12 (or optionally via reboiler 12a). The resulting bottoms fraction is removed via line 14 and usually at least some of it is sewered. A part thereof is passed via line 15 (cooled in exchanger 16) to surge vessel 28. The overhead from the stripper (steam, H28 and NH3) is passed via line 17 to reactor furnace 18 wherein it contacts the hydrocarbon feed fed via line 19. The product resulting from the cracking is passed via line 20 to quench column 211 wherein it contacts recirculated, cooled liquid fed via line 22. The resulting liquid is passed via lines 23 and 15 to quench surge vessel 28 wherein the condensed hydrocarbon phase forms an upper layer. The gaseous overhead from column 21 is passed via line 24 to condenser 25 (quench solution being mixed therein, fed via -line 31) n and then Via line 26 to knock-out pot 27. Aqueous liquid and condensed hydrocarbons are passed from pot 27 via line 34 to vessel 33. The lower aqueous layer is passed from vessel 28 via line 29 to cooler 30' where it is cooled and then via line 22V to column 21. The upper hydrocarbon layer is passed from vessel 28 via lines 32 and 34 to surge vessel 33 wherein two layers separate, the upper hydrocarbon layer being passed via line 46 to hydrocarbon purification. The bottom aqueous layer from vessel 33 is passed via line 13 to stripper 11. From pot 27, vapor is passed via line 35, compressed in compressor 36,

and then via line 37 (cooled in cooler 38) to recovery column 39 wherein it is contacted with fresh water fed via line 44. The hydrocarbon vapor from column 39 is passed via line 40 to hydrocarbon purification. The liquid from. column 39 is recycled via line 41 (cooled in cooler 42) back to this column. A part thereof is passed via lines 43 and 15 to surge vessel 28. Optionally, a part of the stripper bottoms is passed via lines 15 and 45 (cooled in cooler 46) and line 44 as feed to column 39 to replace all or a part of the fresh Water.

The columns are of usual design, and may be of packed, tray or the like types. Additional coolers, scrubbers, and the like may 'be included if desired t-o scrub the liquid and/ or vapor hydrocarbon more completely.

Make-up NH3 and H28 are introduced via line 47 into vessel 28.

In order to indicate still more fully the nature of the present invention the following examples of typical procedures are set forth in which parts and percents 'mean' parts and percents by weights, respectively, unless otherwise indicated, it being understood that these examples are presented as illustrative only and they are not intended to limit the scope of the invention.

Example 1 Using the above described system, a mixture of 2- methyl pentene-Z and Z-methyl pentene-l is cracked at about 650 C., and the resulting reacted gas stream contains about 2.1 wt. percent H28, 3.36% NH3, 36.8% H2O and the remainder a mixture of hydrocarbons, mostly C5 and C3 hydrocarbons. About 8,000 lbs/hr. of this mixture is cooled to 130 F. by contacting with aqueous ammoniacal solution, a part thereof condensing to form a lower aqueous layer, and an upper hydrocarbon layer. The uncondensed part contains hydrocarbons and some H2S and NH3 and this is further condensed by chilling to 60 F. in contact with aqueous ammoniacal solution from the first step, about 44,000 lbs/hr. of vapor being treated with yabout 23,000 lbs/hr. ammoniacal solution. The lresulting hydrocarbon liquid is separated and processed to recover the desired isoprene material.

The aqueous liquid is stripped with steam at a pressure ofvlOO p.s.i.a. to vgive a vapor at about 325 F. containing about 5 wt. percent H28, about 8% NH3 and about 87% H2O. This vapor is fed at about 33,500 lbs/hr. into a cracking reactor along with hydrocarbon feed which is mainly isoprene precursors having six carbon atoms in the molecule, the hydrocarbon feed rate being about 46,000 lbs./hr. The stripper liquid, now essentially free of H2S and NH3, is recycled back to the absorption system.

At the stripping temperature of 325 F., the aqueous solution to the stripper contains 4 Wt. percent H28, the actual concentration depending on the quantity of stripped liquid leaving the column to be recycled. This quantity is chosen, as desired, preferably for most economical operation.

In this system, HZS is provided in a vapor form, at the desired concentration 'and pressure for the catalytic reaction step, and the use of intermediate compressors is completely avoided. In addition, the steam used for stripping the H28 from the absorbing liquid remains in vapor form for use as diluentinstead of being condensed and separated (as it would be in the conventional system), thus providing steam economy.

. The stripped aqueous liquid is recycledV to the absorption fand cooling system where it regains its former concentration of VH28 and NH3. This recycle minimizes or avoids troublesome waste disposal problems.

It is desirable to discard a portion of the aqueous stream, because of build-up of water. Polymers, tars and inorganic residues are conveniently purged from the system thereby. This portion of the stripped liquid may be discarded directly to a sewer, or if necessary to conform to extremely stringent polution regulations, maybe stripped of the trace quantities of H28 still present by an additional steam-stripping step at lower pressure, or treated in a waste disposal unit. In the alternate using direct steam injection to the stripper, a usual reboiler is not required, resulting in economies in capital as Well as operation. However, there is a larger volume of discarded material.

In processes where the SH providing compound and hydrocarbons or other low boiling materials are mixed at low temperatures before entering the heaters and reactors, the vaporized hydrocarbon could be used as the H28 stripping medium, e.g. methane, ethane, propane, 4or the like, instead of steam. If little or no diluent steam were required, the use of the hydrocarbon for stripping would lower the heating cost.

More generally, the scrubbing liquid is Water containing ammonia and H28 (the ratio may vary from 1 to 5 parts by weight of NH3 per part of H28). The H28 concentration in the circulating aqueous streams varies from about 0 up to about 5% by weight, as the lean liquor is enriched, and then stripped and recycled.

In the stripping step, the temperature may be in the range of about 135 to about 180 C. at the corresponding pressure in the range Vof 50 to 150 p.s.i.'a. The system is operable for use with any reactor pressure, and this sets the minimum stripping pressure, for feeding vapor directly to the reactor.

The stripper vapor usually contains about 2 to 10 mol percent of H28, the amount being selected to give the desired H28 concentration in the reactor. The vapor contains H2O and NH3, the ratio of NH3 to H28 being about the same as or somewhat lower than mentioned above. The concentration of NH3 is not critical since it is a diluent as is steam in the reactor.

In view of the foregoing disclosures, variations and modifications thereof will be apparent to one skilled inthe art, and it is intended to include within the invention all such variations and modifications except as do not come within the scope of the appended claims.

What is claimed is:

1. A process for recovering a source of SH radicals from the reaction product of a reaction step carried out in the presence of SH radicals -as catalyst and an inert diluent, and recycling the sulfur-containing material and diluent to said reaction step, which process comprises contacting said reaction product with aqueous ammoniacal solution whereby most of the sulfur-containing material is absorbed by said aqueous solution, separating and stripping the sulfur-rich aqueous solution with the diluent which functions as a volatile inert stripping agent to form a vapor mixture containing the source of 8H radicals together With diluent and ammonia, and passing said vapor mixture to said reaction step.

2. A process of claim 1 w-herein steam is the stripping agent and the stripping step is carried out at an elevated pressure suiciently high to give a recycled vapor mixture at the `desired inlet pressure for the elevated pressure reaction Step.

3. A process of claim 2 wherein the reaction step is cracking an olefin having in the molecule a carbon-to-carbon bond in a position beta to the `double bond, in the presence `of from about 2 to about 10 mol percent of SH radicals.

4. A process of `claim 3 wherein the olen contains six carbon atoms in the molecule.

5. A process of claim 4 wherein the steam stripped aqueous solution is recycled as liquid for contacting the reaction product.

6. A process of claim 5 wherein the recycled stripper liquid is used to cool the reaction product by direct Contact.

7. A process of claim 5 wherin the recycled stripper liquid is used to scrub SH providing material from the reaction product.

8. A process of claim 5 wherein the contacted reaction i product is partly liquid and partly gaseous, and the latter part is partially condensed by cooling in contact with dilute aqueous ammoniacal solution, whereby more sulfur-containing material is removed therefrom, and the resulting ammoniacal solution is included in the material subjected to the steam stripping step.

9. A process of claim 8 wherein the recycled vapor mixture is .at about p.s.i.a., the sulfur-rich aqueous iammoniacal solutions are mixed together and the resulting solution contains not over about 5 wt. lof sulfur-containing material calculated as H28, the vapor containing at least 2 mols percent of sulfur-containing material calculated as H28.

1t). A process of claim 8 wherein the content of SH providing material in the vapor is higher than the content thereof in the aqueous liquid.

11. A process of claim 1t? wherein the vapor recycled contains the concentration of SH used in the cracking step.

References Cited by the Examiner UNITED STATES PATENTS 3,238,270 3/ 1966 Turnquest 260-680 FOREIGN PATENTS 250,488 5/ 1963 Australia.

DELBERT E. GANTZ, Primary Examiner'. G. E. SCHMITKONS, Assistant Examiner. 

1. A PROCESS FOR RECOVERING A SOURCE OF SH RADICALS FROM THE REACTION PRODUCT OF A REACTION STEP CARRIED OUT IN THE PRESENCE OF SH RADICALS AS CATALYST AND AN INERT DILUENT, AND RECYCLING THE SULFUR-CONTAINING MATERIAL AND DILUENT TO SAID REACTION STEP, WHICH PROCESS COMPRISES CONTACTING SAID REACTION PRODUCT WITH AQUEOUS AMMONIACAL SOLUTION WHEREBY MOST OF THE SULFUR-CONTAINING MATERIAL IS ABSORBED BY SAID AQUEOUS SOLUTION, SEPARATING AND STRIPPING THE SULFUR-RICH AQUEOUS SOLUTION WITH THE DILUENT WHICH FUNCTIONS AS A VOLATILE INERT STRIPPING AGENT TO FORM A VAPOR MIXTURE CONTAINING THE SOURCE OF SH RADICALS TOGETHER WITH DILUENT AND AMMONIA, AND PASSING SAID VAPOR MIXTURE TO SAID REACTION STEP. 